Automatic buff pressure setting

ABSTRACT

An improved control circuit for use in a surface finishing machine having a driven surface finishing roll is adapted to establish and maintain a desired working pressure relationship between the finishing roll and a workpiece.

United States Patent Weatherell [54] AUTOMATIC BUFF PRESSURE SETTING 1 May30, 1972 [56] References Cited UNITED STATES PATENTS [72] Invent: James weathere'" Olefin 3,589,072 6/1971 Burt ..51/45 [73] Assignee: Clair Mfg. Co., Inc., Olean, N.Y.

Primary Examiner-William R. Armstrong [22] Fried: Nov. 20, 1970 Attorney Bean & Bean 2] A I. No.: 91,398 1 pp 57 ABSTRACT An improved control circuit for use in a surface finishing 2? machine having a driven surface finishing roll is adapted to l n establish and maintain a desired working pressure relationship [58] Field of Search ..5i/45, 165.01, 165.03, 100, between the finishing roll and a workpiece 5 Claims, 6 Drawing Figures 3wm win E 204- 45 5'0 A 772 TH T 2 1"? 4 5 176; r i fzl =iF L|q F- *Li =il 455 i I64- 774 ZWAYvflu/s THEOTTL' 7,2,0

Al 1 7/? THNK Tfl/Vk EX i/(Pc) 762 198 3W4) VALVE MC- EX K 750 Patented May 30, 1972 3,665,647

2 Sheets-Sheet 1 @Mwm A T TOR/VEYS Patented May 30, 1972 3,665,647

2 Sheets-Sheet k,

E9 464 4'5. L55 df 126: W L54 1 N VEN'I UR.

726 724 /h 724 JfiMES m WEHTHERELL I2 720 wim qlm AUTOMATIC BUFF PRESSURE SETTING BACKGROUND AND SUMMARY OF INVENTION The present invention relates to improvements in surface finishing machines and more particularly to improvements in finishing machines of the type disclosed in commonly assigned co-pending U.S. Pat. No. 3,589,072 issued June 29, 1971.

In the above enumerated application, there is disclosed a finishing machine comprising a base for supporting a workpiece carrying table motivated to reciprocate workpieces relative to a finishing roll, which is mounted for movement toward and away from the plane of reciprocation of the workpiece, and a control system for regulating movements of the finishing roll with a view to producing substantially uniform working pressure relationships between the finishing roll and the surface of the workpiece regardless of its contour.

More specifically, the control system of the above mentioned application includes a fluid cylinder enclosing a movable piston coupled to the finishing roll in order to effect movement thereof towards or away from the workpiece; a balance control circuit for applying relatively high pressure air to the cylinder sufficient to counterbalance the weight of the finishing roll and to normally move same away from the workpiece; a finishing roll pressure circuit for selectively applying rela tively low pressure air to an opposite end of the cylinder in order to partially counterbalance the high pressure air and thereby permit a desired portion of the weight of the finishing roll to be applied as pressure to the workpiece; a reduce force circuit for effecting a decrease or reduction in the pressure of the low pressure air; and means to control operation of the reduced forced circuit in accordance with changes in profile of the workpiece surface.

One difficulty experienced in finishing machines employing a control circuit of the type disclosed in the above-mentioned application is that movement of the finishing roll towards the workpiece at the initiation of an operational cycle is sluggish or slow, particularly where medium to low finishing pressures are to be applied to the workpiece. Moreover, in use it has been found that under certain pressure conditions it is necessary for the operator of the machine to manually force the finishing roll downwardly into engagement with the workpiece.

The present invention is directed towards an arrangement for momentarily interconnecting the counterbalance circuit with the finishing roll pressure control circuit in order to make available the full weight of the finishing roll for the purpose of driving the finishing roll downwardly into association with the workpiece; such interconnection being thereafter interrupted in order to maintain a desired work-pressure relationship between the finishing roll and the surface of the workpiece.

DRAWINGS The nature and mode of the present invention will now be more fully described in the following detailed description taken with the accompanying drawings wherein:

FIG. 1 is a front elevational view of a finishing machine in which the present invention has utility;

FIG. 2 is a left hand elevational view of the machine shown in FIG. 1;

FIG. 3 is a fragmentary sectional view taken generally along the line 33 in FIG. 2;

FIG. 4 shows the control system of the finishing machine modified in accordance with the present invention;

FIG. 4a is a sectional view taken generally along the line 4a-4a in FIG. 4; and

FIG. 4b is a sectional view taken generallyalong the line 4b4bin FIG. 4.

DETAILED DESCRIPTION It will be understood that the improved control system of the present invention, which is generally designated as in FIG. 4, has utility in finishing machines or apparatus of the type generally designated as 12 in accompanying FIGS. 1-3.

The construction of finishing machine 12 is described in detail in commonly assigned co-pending U.S. Pat. No. 3,589,072 issued June 29, 1971. However, for reference purposes, finishing machine 12 will now be described as generally including a base formed from a pair of standards 2020 arranged to support a bed plate 21 on which is supported a pair of upright pedestals 22-22 at its opposite sides. Pedestals 22-22 mount in vertically adjustable relation a pair of bearing blocks, 25, 25, which carry therebetween a horizontally disposed, non-rotatably mounted bridge shaft 26. Shaft 26 carries by means of stirrups 27, 27 a rotatable lower shaft 28, which in turn supports arms 30, 32 and 33. Arms 30, 32 and 33 rotatably support drive shaft unit 34 for a finishing roll 35, which may be a grinding, abrading, brushing or polishing or buffing roll or the like, as may be required for any given instance of use. Drive shaft unit 34 is preferably driven by an electric motor, not shown, through means of a belt 36, a pulley 37, an idler roll 38 riding freely on shaft 28, and a drive belt 39. An amp meter, not shown, is provided to appraise an operator of the load imposed on the finishing roll drive motor.

A carriage for mounting a workpiece 45 to be processed is shown as generally including a platform 50, which is mounted on base plate 21 by means of slideways 52 for horizontal reciprocating movement in a direction transversely of roll 35 between pedestals 22. A hydraulic-piston cylinder unit 55 is mounted under base plate 21, such that its piston rod 56 extends in the direction of platfonn travel for connection with a bracket 57 depending from platform 50. Upright brackets 58 extend from the comer portions of platform 50 and support in a suitably elevated relation thereon a support table 60 on which workpieces 45 are to be disposed during the finishing operations. As will be apparent from viewing FIGS. 1 and 2, upon reciprocation of table 60 by means of cylinder 55, workpieces 45 will be passed back and forth beneath and in finishing contact with finishing roll 35. Also, it will be understood that the workpieces to be processed may either be in the form of fiat sheet stock material, as indicated in FIG. 1, or be contoured, as indicated in FIG. 2. As is conventional, the extent of reciprocation of platform 50 and thus table 60 may be readily controlled by means of mechanism, not shown, controlled by hand wheel 70. Also, as is conventional, machine 12 may include controls for causing axial reciprocation of finishing roll 35 coincident with its rotation in order to avoid production 0 graining effects or the like on the finished surface of workpiece 45.

The elevational position of finishing'roll 35 relative to a workpiece supported on table 60 may be regulated by means of the combination hand and pneumatic piston cylinder control unit 100, shown in FIGS. 2 and 4, which forms a part of control system 10, shown only in FIG. 4. Unit includes a piston rod 102, which is threaded at one end thereof to a nut bearing 104, which is in turn carried by a crank arm 106 keyed for rotation with shaft 28. A hand wheel 108 is fitted to the upper end of rod 102 in order to facilitate manual adjustment thereof relative to nut bearing 104. At its lower end rod 102 carries a piston head 110, which rides within fluid cylinder 112, mounted on the machine base, as by a pivot connection 114.

It will be understood that hand wheel 108 may be employed to make fine adjustments of the positioning of piston head 102 within cylinder 112 as required by slight variations in thickness. of workpiece 45, whereas major adjustments may be made by adjusting table 60 relative to platform 50 by suitable means, not shown.

To adapt machine 12 for processing workpieces having marked surface contour variations, there is provided a control unit generally designated as in FIGS. 3-4B. Control unit 120 includes a pair of slide rods 122, which are fixed to platform 50, such as to extend lengthwise thereof in the direction of platform reciprocation. A plurality of control blocks 124 are slideably mounted on rails 122 for adjustment'lengthwise thereof in order to permit the control blocks to be arranged substantially coincident with marked changes in contour of workpiece 45 in the manner best shown in FIG. 4. Each of control blocks 162 is provided with an operator or finger portion 126, which are set so as to extend alternately into registry with a pair of suitable sensors LS4 and LS5, which are positionally fixed to the base of machine 12. Thus, sensors LS4 and LS5 will be alternately actuated by operators 126 substantially coincident with the change in contour of the workpiece being reciprocated relative to finishing roll 35.

Now referring specifically to FIG. 4, it will be understood that control system generally includes a plurality of fluid circuits including a counterbalance circuit 130, a work pressure control circuit 140, a reduce force circuit 150 and a roll lowering or coupling circuit 160.

Counterbalance circuit 130 is shown as including a manually adjustable pressure regulator 162 through which air under pressure is passed to a surge tank 164 and subsequently directed to the upper end of cylinder 112 through line 166 having as desired a suitable pressure or flow control valve 168. Pressure control circuit 140 is shown as including a manually adjustable pressure regulator 172 through which air under pressure is passed to a surge tank 174 and subsequently to the lower end of cylinder 112 through line 176, a dual pressure shuttle valve 178, line 180, three way valve 182 and line 184. As required, a suitable pressure or flow control valve 186 may be disposed in line 184 between three way valve 182 and the bottom end of cylinder 112. It will be understood that three way valve 182 is normally in a closed condition, wherein line 180 is blocked and line 184 vented to the atmosphere. Three way valve 182 is movable into the position shown in FIG. 4 upon operation of a solenoid operator A, which is in turn actuated by means of a suitable electric circuit including operating button shown at 190 only in FIG. 1.

Reduce force circuit 150 is shown as including a line 192, which is branched from line 184, two way valve 194, and line 196, which opens to the atmosphere through a manually adjustable throttle valve 198. It will be understood that two way valve 194 includes a solenoid operator F whose condition is determined by which of sensors LS4 and LS5 have been most recently engaged by one of operator fingers 126. Thus, two way valve 194 is in closed condition subsequent to operation of sensor LS5, but before operation of sensor LS4, whereas the valve is in its open condition subsequent to operation of sensor LS4, but before operation of sensor LS5.

Coupling circuit 160 includes a line 202, which is branched from line 166, a three way valve 204, and line 206 in addition to shuttle valve 178 mentioned above. It will be understood that three way valve 204 is normally in its closed condition wherein branch line 202 is blocked and line 206 vented to atmosphere, but is movable to place branch line 202 in flow communication with line 206 upon energization of solenoid operator E. Solenoid E, which may be conveniently arranged in parallel circuit with solenoid A, is automatically energized upon initiation of the operation of the machine by depressing control button 190, but is substantially immediately deenergized by a suitable time delay device 208.

Shuttle valve 178 is characterized as having a cavity 210 into which each of lines 176, 180 and 206 is arranged for fluid communication. A flexible generally disc-shaped diaphragm 212 is loosely fitted within cavity 210. It will be understood that the construction is such that when air pressure in line 176 exceeds that in line 206, diaphragm 212 is forced to block line 206 and air from line 176 passes freely about the edges of the diaphragm into line 180. However, when the air pressure in line 206, and/or line 180, exceeds that in line 176, diaphragm 212 is forced to move and effect blocking of the outlet of line 176 and air is passed directly between lines 206 and 180.

Finishing machine 12 is initially set up by adjusting hand wheel 108 to insure that piston head 110 will not abut against the upper end of cylinder 112 when finishing roll 35 is fully worn and in its full down position. Thereafter, the operator will adjust regulator 162 in order to apply high pressure air through line 166 to the upper end of cylinder 112, which is slightly above that necessary to balance and compensate for the weight of the finishing roll, such that piston head 102 is biased downwardly within cylinder 112 and the finishing roll raised into its phantom line position shown in FIG. 4. With a machine of the type illustrated in the drawings and the finishing roll being in the form of a conventionally sized buffing roll, a raise roll pressure of about 40 psi is employed.

Setting of regulator 162 is carried out with valves 182, 194 and 204 in their normally closed conditions, such as to block passage of roll raise air through branch line 202 to shuttle valve 178, and to vent the lower end of cylinder 112 to atmosphere through valve 182.

Thereafter, the operator initially adjusts work pressure regulator 172 in order to apply a pressure of normally between about 10 and 20 psi to line 176. The operator then activates the machine start button, which effects energization of solenoids A and E by means of suitable circuitry, not shown. As a result, high pressure air is immediately passed from branch line 202 through valve 204, line 206, valve 178, line 180, valve 182 and line 184 into the lower end of cylinder 112, whereupon the pressure across piston head is substantially equalized and the full weight of roll 35 pemiitted to initiate movement of the finishing roll downwardly into engagement with workpiece 45. At this point, work pressure line 176 is closed by diaphragm 212, due to the presence of 40 psi air in line 206. Immediately thereafter, that is within about one second, solenoid E is deenergized by operation of a time delayed device 208, whereby returning valve 204 to its normal condition, wherein branch line 202 is blocked and line 206, and thus lines 180, 184 and the lower end of cylinder 112 are exhausted to the atmosphere through valve 204. As the pressure in line 206 and thus the lower end of cylinder 112 falls to about the 10-20 psi work pressure in line 176, diaphragm 212 is substantially immediately forced to close off line 206 and place lines 176 and 180 in communication in order to establish the desired work pressure in the lower end of cylinder 112. Operation of the shuttle valve occurs at such a rate that work pressure is normally applied to the lower end of cylinder 112 before the finishing roll is forced from engagement with the workpiece by the 40 psi raise pressure continuously present in the upper end of the cylinder 112, during the above described operation. It will be understood that the application of high pressure air to the lower end of the cylinder is preferably only momentary, that is, only of sufficient duration to pennit the unbalance weight of the finishing roll to innate movement thereof towards the workpiece. The almost immediate reduction of the pressure in the lower end of the cylinder to the previously set work pressure serving to cushion contact between the falling finishing roll and the workpiece in order to prevent any damage to the latter.

Regulator 172 is then manually adjusted by the operator to vary the work pressure, as determined by visual inspection of the finish produced on the surface of the workpiece. When the workpiece is flat sheet material or only slightly contoured the above operations complete setting of the machine. Finishing roll 35 may be raised to its phantom line position, shown in FIG. 4, whenever it is desired to remove or introduce a workpiece by deenergizing solenoid A so as to return valve 182 to its initial closed condition, wherein line 180 is blocked and line 184 is vented to the atmosphere.

It will be understood that after regulator 172 has been set by the operator to provide a desired surface finish, substantially identical pressure will be subsequently maintained on the workpiece regardless of the degree of wear to which finishing roll 35 is subjected. This is to be compared with arrangements, wherein continued adjustment of hand wheel 108 was necessary to compensate for wear of the finishing roll. Setting up of the finishing machine to accommodate in regularly shaped workpieces, ie. those having vertical inclines of more than about 30, is identical to that described above, except that during the final adjustment of work pressure regulator the operator makes a note of the work performed by the finishing roll driving motor, not shown, as indicated by an associated amp meter, also not shown. In this respect, it will be understood that the amp meter reading of the drive motor will remain substantially constant when the finishing roll is processing either a flat workpiece or moving down an inclined surface of a contoured workpiece, ie. between points 2-3 and 45 in FIG. 4, due to the fact that work pressure is maintained at a constant value. However, when the finishing roll is forced to move up an incline of greater than about 30 ie. between points 1-2 and 3-4 in FIG. 4, it has been observed that the finishing roll tends to dig into the workpiece, such that the workpiece is finished to a greater extent than desired and thus the drive motor is subjected to an increased load. Thus, when setting up for contoured work, the operator adjusts the throttle valve 198, when two way valve 194 is in its open condition to bleed air off from the lower end of cylinder 112 in an amount necessary to obtain an amp meter reading identical to that previously observed in the initial setting up of the work pressure.

During operation, solenoid F will normally remain deenergized in order to maintain two way valve 194 in its normally closed condition. When sensor LS4 is engaged by an operator finger 126, solenoid F is operated to place two way valve 194 in its open condition and permit a desired amount of air to bleed off to atmosphere as determined by the setting of throttle valve 198. As a result, finishing roll 35 is freed to permit movement thereof up the incline without performing excessive finishing action on the workpiece. Coincident with the positioning of the finishing roll at the top of the incline, an operator finger 126 is positioned in engagement with sensor LS5 to effect deenergization of solenoid F and the resultant closing of two way valve 194. Simultaneously therewith, the normal work pressure is reapplied to the lower end of cylinder 112 in order to force finishing roll 35 to move down the incline while exerting the desired work pressure. Opening and closing of two way valve 194 will continue as determined by the preset positioning of operator finger 126. Of course, it will be understood that the location of operator fingers 126 may be adjusted as desired to provide for a desired lead and lag operation of the pressure control system and that the pressure bleed ofi and build up sequences may be readily regulated as to timing and degree, so as to accommodate for finishing of any irregularly or regularly contoured workpiece.

While the control system of the present invention has been described with particular reference to finishing machines of the type disclosed in US. Pat. No. 3,589,072 issued June 29, 1971, it will be appreciated that it possesses utility in diverse types of finishing apparatus wherein it is desired to lower a finishing implement into engagement with a workpiece.

lclaim:

1. In a workpiece surface finishing apparatus including a base, means to support a workpiece on said base, a finishing roll mounted on said base for movement towards and away from said workpiece between a first position and a second workpiece engaging position, and a control system regulating movement of said finishing roll relative to said workpiece, the improvement wherein said control system comprises in combination:

a fluid cylinder;

a piston movable relatively within said cylinder, one of said cylinder and piston being fixed to said base and the other thereof being coupled to said roll, said piston when movable in one direction relative to said cylinder tending to raise said roll upwardly away from said workpiece into said first position and when movable in an opposite direction permitting the weight of said roll to move same into said second position; counterbalance circuit including a source of air under relatively high pressure and means connected into one end of said cylinder for continuously applying air from said high pressure source to said one end of said cylinder, said high pressure tending to counterbalance the weight of said roll and to move said piston in said one direction;

a finishing roll pressure control circuit including a source of air under relatively low pressure and means connected into another end of said cylinder, said control circuit means having first and second conditions, said control circuit means when in said first condition blocking said low pressure source and venting said other end of said cylinder to atmosphere to permit said high pressure air applied to said one end of said cylinder to maintain said roll in said first position, said control circuit means when in said second condition normally connecting said lower pressure source to said other end of said cylinder, said low pressure when applied tending to partially counterbalance said high pressure applied to said one end of said cylinder such that the difference between said high and low pressures determines the pressure of said finishing roll on said workpiece when in said second position; and

a finishing roll lowering circuit for momentarily coupling said counterbalance circuit to said control circuit when said control circuit means is in said second condition thereby to momentarily apply said high pressure air to said other end of said cylinder to substantially counterbalance said high pressure air continuously applied to said one end of said cylinder to permit substantially the full weight of said roll to initiate movement thereof from said first into said second position.

2. In a workpiece surface finishing apparatus including a base, means to support a workpiece on said base, a finishing roll mounted on said base for movement towards and away from said workpiece between a first position and second workpiece engaging position, and a control system regulating movement of said finishing roll relative to said workpiece, the improvement wherein said control system comprises in combination:

a fluid cylinder;

a piston movable relatively within said cylinder, one of said cylinder and piston being fixed to said base and the other thereof being coupled to said roll, said piston when movable in one direction relative to said cylinder tending to raise said roll upwardly away from said workpiece into said first position and when movable in an opposite direction pennitting the weight of said roll to move same into said second position; counterbalance circuit including a source of air under relatively high pressure and first conduit means' connected into one end of said cylinder for continuously applying air from said source to said one end of said cylinder, said high pressure tending to counterbalance the weight of said roll and to move said piston in said one direction;

a finishing roll pressure control circuit including a source of air under relatively low pressure, second conduit means connected to said source of relatively low pressure, third conduit means, first valve means having a normal first condition and a second condition, and fourth conduit means connected into said other end of said cylinder, said first valve means when in said first condition venting said other end of said cylinder to atmosphere through said fourth conduit means and blocking said third conduit means, said first valve means when in said second condition placing said third and fourth conduit means in fluid communication;

a roll lowering circuit including branch conduit means connected into said first conduit means, a second valve means having a normal first condition and a second condition, fifth conduit means and shuttle valve means, said second, third and fifth conduit means being connected into said shuttle valve means, said shuttle valve means blocking said second conduit means and connecting said fifth and third conduit means in fluid communication when air pressure within either thereof exceeds air pressure in said second conduit means, said shuttle valve means blocking said fifth conduit means and connecting said second and third conduit means in fluid communication when air pressure in said second conduit means exceeds air pressure in said fifth conduit means, said second valve means when in said first condition blocking said branch conduit means and connecting said fifth conduit means to atmosphere and when in said second condition placing said branch conduit means and said fifth conduit means in flow communication; and

control means for moving said first and second valve means from said first into said second conditions thereof, said control means of said second valve means including delay means for substantially immediately returning said second valve means from said second into said first condition thereof.

3. A method of lowering a finishing roll into contact with a workpiece and thereafter maintaining a substantially uniform pressure relationship between said finishing roll and said workpiece comprising the steps of:

coupling a fluid cylinder-piston unit to said finishing roll such that movement of its piston in one direction relative to its cylinder tends to raise said roll upwardly away from said workpiece into a first position remote therefrom, whereas relative movement of said piston in an opposite direction pennits the weight of said roll to move same into a second workpiece engaging position;

continuously biasing said unit to move said piston in said one direction for normally maintaining said roll in said first position;

momentarily introducing air at high pressure into said cylinder to substantially counterbalance said bias thereby to permit substantially the full weight of said roll to initiate movement thereof towards said second position; and

reducing said high pressure to a relatively low pressure during the period of movement of said roll toward and into said second position, the difference between said bias and said low pressure determining the working pressure between said roll and said workpiece.

4. A method of lowering a finishing roll into contact with a workpiece and thereafter maintaining a substantially uniform pressure relationship between said finishing roll and said workpiece comprising the steps of:

coupling a fluid cylinder piston unit to said finishing roll such that movement of its piston in one direction relative to its cylinder tends to raise said roll upwardly away from said workpiece into a first position remote therefrom, whereas relative movement of said piston in an opposite direction permits the weight of said roll to move sarne into a second workpiece engaging position;

continuously introducing relatively high pressure air into one end of said cylinder whereby to move said piston in said one direction and normally maintain said roll in said first position thereof;

momentarily introducing air at said high pressure into another end of said cylinder to substantially balance air pressure across said piston thereby to permit substantially the full weight of said roll to initiate movement thereof towards said second position; and

reducing said relatively high pressure applied to said other end of said cylinder to a relatively low pressure during the period of movement of said roll toward and into said second position, the difference between said continuously introduced relatively high and relatively low pressures determining the working pressure between said roll and said workpiece.

5. A method of lowering a finishing roll into contact with a workpiece and thereafter maintaining a substantially uniform pressure relationship between said finishing roll and said workpiece comprising the steps of:

coupling a fluid cylinder-piston unit to said finishing roll such that movement of its piston in one direction relative to its cylinder tends to raise said roll upwardly away from said workpiece into a first position remote therefrom, whereas relative movement of said piston in an opposite direction permits the weight of said roll to move same into a second workpiece engaging position;

continuously introducing relatively high pressure air into one end of said cylinder whereby to move said piston in said one direction and normally maintain said roll in said first position thereof;

introducing air at said high pressure into another end of said cylinder to substantially balance air pressure across said piston thereby to permit substantially the full weight of said roll to initiate movement thereof towards said second position;

venting said other end of said cylinder to atmosphere; and

terminating the venting of said other end of said cylinder and connecting same to a source of relatively low pressure when the pressure within said other end of said cylinder is reduced by venting to a pressure substantially corresponding to the pressure of said low pressure source, the difference between said continuously introduced relatively high and relatively low pressures determining the working pressure between said roll and said workpiece. 

1. In a workpiece surface finishing apparatus including a base, means to support a workpiece on said base, a finishing roll mounted on said base for movement towards and away from said workpiece between a first position and a second workpiece engaging position, and a control system regulating movement of said finishing roll relative to said workpiece, the improvement wherein said control system comprises in combination: a fluid cylinder; a piston movable relatively within said cylinder, one of said cylinder and piston being fixed to said base and the other thereof being coupled to said roll, said piston when movable in one direction relative to said cylinder tending to raise said roll upwardly away from said workpiece into said first position and when movable in an opposite direction permitting the weight of said roll to move same into said second position; a counterbalance circuit including a source of air under relatively high pressure and means connected into one end of said cylinder for continuously applying air from said high pressure source to said one end of said cylinder, said high pressure tending to counterbalance the weight of said roll and to move said piston in said one direction; a finishing roll pressure control circuit including a source of air under relatively low pressure and means connected into another end of said cylinder, said control circuit means having first and second conditions, said control circuit means when in said first condition blocking said low pressure source and venting said other end of said cylinder to atmosphere to permit said high pressure air applied to said one end of said cylinder to maintain said roll in said first position, said control circuit means when in said second condition normally connecting said lower pressure source to said other end of said cylinder, said low pressure when applied tending to partially counterbalance said high pressure applied to said one end of said cylinder such that the difference between said high and low pressures determines the pressure of said finishing roll on said workpiece when in said second position; and a finishing roll lowering circuit for momentarily coupling said counterbalance circuit to said control circuit when said control circuit means is in said second condition thereby to momentarily apply said high pressure air to said other end of said cylinder to substantially counterbalance said high pressure air continuously applied to said one end of said cylinder to permit substantially the full weight of said roll to initiate movement thereof from said first into said second position.
 2. In a workpiece surface finishing apparatus including a base, means to support a workpiece on said base, a finishing roll mounted on said base for movement towards and away from said workpiece between a first position and second workpiece engaging position, and a control system regulating movement of said finishing roll relative to said workpiece, the improvement wherein said control system comprises in combination: a fluid cylinder; a piston movable relatively within said cylinder, one of said cylinder and piston being fixed to said base and the other thereof being coupled to said roll, said piston when movable in one direction relative to said cylinder tending to raise said roll upwardly away from said workpiece into said first position and when movable in an opposite direction permitting the weight of said roll to move same into said second position; a counterbalance circuit including a source of air under relatively high pressure and first conduit means connected into one end of said cylinder for continuously applying air from said source to said one end of said cylinder, said high pressure tending to counterbalance the weight of said roll and to move said piston in said one direction; a finishing roll pressure control circuit including a source of air under relatively low pressure, second conduit means connected to said source of relatively low pressure, third conduit means, first valve means having a normal first condition and a second condition, and fourth conduit means connected into said other end of said cylinder, said first valve means when in said first condition venting said other end of said cylinder to atmosphere through said fourth conduit means and blocking said third conduit means, said first valve means when in said second condition placing said third and fourth conduit means in fluid communication; a roll lowering circuit including branch conduit means connected into said first conduit means, a second valve means having a normal first condition and a second condition, fifth conduit means and shuttle valve means, said second, third and fifth conduit means being connected into said shuttle valve means, said shuttle valve means blocking said second conduit means and connecting said fifth and third conduit means in fluid communication when air pressure within either thereof exceeds air pressure in said second conduit means, said shuttle valve means blocking said fifth conduit means and connecting said second and third conduit means in fluid communication when air pressure in said second conduit means exceeds air pressure in said fifth conduit means, said second valve means when in said first condition blocking said branch conduit means and connecting said fifth conduit means to atmosphere and when in said second condition placing said branch conduit means and said fifth conduit means in flow communication; and control means for moving said first and second valve means from said first into said second conditions thereof, said control means of said second valve means including delay means for substantially immediately returning said second valve means from said second into said first condition thereof.
 3. A method of lowering a finishing roll into contact with a workpiece and thereafter maintaining a substantially uniform pressure relationship between said finishing roll and said workpiece comprising the steps of: coupling a fluid cylinder-piston unit to said finishing roll such that movement of its piston in one direction relative to its cylinder tends to raise said roll upwardly away from said workpiece into a first position remote therefrom, whereas relative movement of said piston in an opposite direction permits the weight of said roll to move same into a second workpiece engaging position; continuously biasing said unit to move said piston in said one direction for normally maintaining said roll in said first position; momentarily introducing air at high pressure into said cylinder to substantially counterbalance said bias thereby to permit substantially the full weight of said roll to initiate movement thereof towards said second position; and reducing said high pressure to a relatively low pressure during the period of movement of said roll toward and into said second position, the difference between said bias and said low pressure determining the working pressure between said roll and said workpiece.
 4. A method of lowering a finishing roll into contact with a workpiece and thereafter maintaining a substantially uniform pressure relationship between said finishing roll and said workpiece comprising the steps of: coupling a fluid cylinder-piston unit to said finishing roll such that movement of its piston in one direction relative to its cylinder tends to raise said roll upwardly away from said workpiece into a first position remote therefrom, whereas rElative movement of said piston in an opposite direction permits the weight of said roll to move same into a second workpiece engaging position; continuously introducing relatively high pressure air into one end of said cylinder whereby to move said piston in said one direction and normally maintain said roll in said first position thereof; momentarily introducing air at said high pressure into another end of said cylinder to substantially balance air pressure across said piston thereby to permit substantially the full weight of said roll to initiate movement thereof towards said second position; and reducing said relatively high pressure applied to said other end of said cylinder to a relatively low pressure during the period of movement of said roll toward and into said second position, the difference between said continuously introduced relatively high and relatively low pressures determining the working pressure between said roll and said workpiece.
 5. A method of lowering a finishing roll into contact with a workpiece and thereafter maintaining a substantially uniform pressure relationship between said finishing roll and said workpiece comprising the steps of: coupling a fluid cylinder-piston unit to said finishing roll such that movement of its piston in one direction relative to its cylinder tends to raise said roll upwardly away from said workpiece into a first position remote therefrom, whereas relative movement of said piston in an opposite direction permits the weight of said roll to move same into a second workpiece engaging position; continuously introducing relatively high pressure air into one end of said cylinder whereby to move said piston in said one direction and normally maintain said roll in said first position thereof; introducing air at said high pressure into another end of said cylinder to substantially balance air pressure across said piston thereby to permit substantially the full weight of said roll to initiate movement thereof towards said second position; venting said other end of said cylinder to atmosphere; and terminating the venting of said other end of said cylinder and connecting same to a source of relatively low pressure when the pressure within said other end of said cylinder is reduced by venting to a pressure substantially corresponding to the pressure of said low pressure source, the difference between said continuously introduced relatively high and relatively low pressures determining the working pressure between said roll and said workpiece. 